Ventilators



E. FIELDING June 4, 1968 VENT ILATORS 5 Sheets-Sheet 1 Filed May 18,1966 E. FIELDING VENT ILATORS 3 Sheets-Sheet 2 Filed May 18, 1966 MIITFu r n --.f 1] 2 .7 :L 5` u. f

mv.June 4, 1968 E. Flr-:LDING 3,386,368

VENT ILATORS Filed May 1s,l 196e s' sheets-shea s United States Patent O3,386,368 VENTILATORS Eric Fielding, Cosham, England, assignor to ColtVentilation and Heating Limited Filed May 1S, 1966, Ser. No. 551,118Claims priority, application Great Britain, May 19, 1965, 21,120/65 19Claims. (Cl. 98-43) ABSTRACT F THE DESCLOSURE The disclosure relates toa motor operated discharge ventilator having a discharge fan impellerand fan casing wherein the impeller blades closely conform to the shapeof a trumpet shaped discharge outlet so that in operation air leavingthe trailing edges of the impeller blades has a large component ofmovement substantially radially outwardly of the impeller.

This invention relates to ventilators and is concerned with powereddischarge ventilators of the kind including an extraction fan forpromoting a flow of air through the ventilator, and for use mounted on abounding structure (e.g.4a wall or roof) of a space from which air is tobe withdrawn by the ventilator.

Powered discharge ventilators are known employing an extraction fan ofthe axial flow type wherein the fan has a cylindrical casing closelysurrounding the fan impeller and in which the air enters and leaves theimpeller in a direction substantially parallel to the axis of rotationof the impeller, the air being confined and guided by the cylindricalcasing both on entering and leaving the impeller.

The volume of air delivered by such a fan depends inter alia on thediameter of the impeller and this in turn governs the diameter of thecasing and accordingly the overall size of the ventilator.

Broadly, according to the present invention, there is provided a powereddischarge ventilator including an extraction fan having an impeller anda fan casing and in which the tips of the impeller blades are closelysurrounded by and conform to the shape of a trumpet mouthing portion ofthe casing the arrangement being such that in operation of theventilator, air is free to leave the tips of the trailing edges of theimpeller blades with a large component of movement substantiallyradially outwardly of the impeller. Preferably the trumpet mouthingportion forms a smooth downstream continuation of an upstream portion ofthe casing leading air to the fan impeller, the impeller having a meansdiameter greater than the diameter of said upstream casing portionimmediately upstream of the impeller.

By placing the impeller in a trumpet mouthing portion of the casing onemay increase the effective diameter of the impeller compared with theimpeller of a conventional ventilator employing an axial ow fan having acasing of diameter equal for example to said upstream casing portionimmediately upstream of the impeller and in this way the veintilator maybe made to shift more air for a given speed of rotation of the impellerWithout increasing the overall size of the ventilator.

It has also been found that with the arrangement according to theinvention the performance of the ventilator against added resistanceupstream of the fan, for example by making the ventilator draw its airthrough a long length of ducting with bends, is improved compared withthe conventional equivalent ventilator described in the precedingparagraph. It is believed that this arises, using an axial liowimpeller, because the stall "ice point on the characteristicpressure/volume curve is further removed from the zone of maximum volumeof operation of the fan whereby the ventilator has a wider range ofuseful operation.

A still further advantage which is achieved using the arrangementaccording to the invention is that there is a noticeable reduction ofnoise compared with the conventional ventilator employing an axial flowfan and per forming an equivalent duty.

The impeller employed according to the invention may be an axial flowimpeller having aerofoil sectioned blades or a propeller fan impellerhaving sheet form blades or a mixed flow fan impeller in which theblades are developed along a conical or a spherical hub surface. Thelatter form of impeller makes optimum use of the regime provided for itis a ventilator according to the present invention in so far as thecompound curvature of the impeller `blades imparts a greater radialcomponent of movement to the airstream and the performance against addedresistance upstream is still further improved.

The fact that air escapes substantially radially outwardly of theimpeller in a powered discharge ventilator according to this inventionis of particular significance when the invention is applied to a powereddischarge ventilator having means preventing direct whether entrythrough the ventilator since such means requires the airstream beingdischarged by the ventilator to be turned through a large angle and inan arrangement according to the invention the air is turned in part atleast during its passage through the impeller.

An equivalent elfect can be course be achieved using a centrifugal fan,in which case the air is turned through a full when passing through theimpeller.

Powered discharge ventilators employing centrifugal fans are well known.These are used where it is required to develop a high pressure toovercome resistance to the ow of air upstream of the ventilator and inaccordance with the known characteristics of centrifugal fans this isachieved at the expense of the free inlet volume handled by theventilator. Accordingly, to obtain a given volume output of air withfree inlet, a bigger fan with a greater power requirement is necessaryusing a centrifugal fan instead of an axial ow fan and powered dischargecentilators having centrifugal fans tend to take the form of large, highprofile units and such unit, employed as roof extract ventilators, areoften objectionable for purely aesthetic reasons.

According to a feature of the present invention the trumpet mouthingportion of the fan casing is confronted by a member presenting, towardsthe impeller a substantially flat coi-extensive surface disposed normalto the axis of rotation of the impeller, the trumpet mouthing portionhaving an inner wall surface which turns through `a full right angle,and said surface of `said member being predeterminedly spaced from thecrown of the trumpet mouthing portion, at least in use of theventilator, so as to constrain or assist in constraining air to ilowsmoothly round the inner wall surface of the trumpet mouthing portion tothe crown thereof without breaking away from the surface.y

By this arrangement it has been found tha-t the airstream discharged bythe impeller may be turned through 90 in a very eicient manner and in asmall height, `and this materially assists in the production of a roofextract ventilator of low profile.

Without the flat confronting member, the airstream would break away fromthe inner wall surface of a trumpet mouthing turning through such alarge angle and a region of turbulence would be created towards thecrown of the trumpet mouthing portion. By correctly spacing theconfronting member from the trumpet mouth- Iing however it has beenfound that such break away may be prevented. Accordingly it is ensuredthat the tips of impeller blades positioned weil forward in the trumpetmouthing with their trailing edge-s llyin-g as close as is practical tothe plane of the crown of t-he trumpet mouthing portion, operate whollyin a region of streamline ow so as to produce useful thrust at theoptimum blade sections, i.e. those operating at the greatest radius.This enables a maximum increase, as previously discussed, of effectiveimpeller diameter to be achieve-d and therefore the maximum increase ofperformance, again as previously discussed, compared with the equivalentconventional ventilator employing an axial flow fan.

According to a further feature of the present invention the ventilatormay include means presenting confronting surfaces one as a peripheralextension of said surface of said member and the other as a peripheralextension of the inner wall surface of the trumpet mouthing portion,said confronting surfaces together vdefining a confined air dischargepassage for streamline flow of the extracted air the through liow areaof which progressively increases to an air discharge outlet from saidpassage to the outside atmosphere.

By means of this feature the velocity of the discharging airstream isprogressively reduced as the airstream passes through the dischargepassage and the kinetic energy of the airstream is reconverted topotential energy, the static pressure in the discharge passageincreasing progressively towards the outlet. Additionally, sincestreamline ow is maintained, the losses are kept small. By slowing theair velocity at discharge as far as is practical within the dimensionallimits required for the ventilator, the loss of the kinetic energy ofthe discharging airstream may be kept down and the eiiciency of theventilator considerably improved.

By utilising a discharge passa-ge defined between confronting surfacesdirected radially of the axis of rotation of the impeller, the maximumincrease in through-flow are-a of the passage with the smallest radialenlargement of the ventilator may 'be achieved in a ventilator of lowprofile.

To prevent weather entry through the air discharge outlet of the airdischarge passage the air discharge outlet is conveniently offsetrelative to the inlet to said air discharge passage in the direction ofthe axis of rotation of the impeller, and in this case said confrontingsurfaces are gently curved between said inlet and said outlet to preventbreak away of the discharging airstream therefrom.

A specific embodiment of the present invention is hereinafter describedby way of example with reference to the accompanying drawings in which:

iFIG. 1 is a cross-sectional side elevation of a powered discharge roofventilator according to the present invention, on line 1-1 in FIG. 2.

FIG. 2 is a plan view of the ventilator shown in FIG. 1, and

FIG. 3 is a cross-section on line 3--3 in FIG. 1.

The ventilator illustrated may be positioned on a flat or on a pitchedroof. The ventilator comprises a base which would be flashed to the roofin any known or convenient manner. The base 10 is gener-ally rectangularin plan but with rounded corners and is of sheet metal construction inthis example, opening upwardly to a circular sectioned throat portion 12of a fan casing being connected therewith by means of a smoothlyupwardly convergent transitional portion 11 of the casing. The throatportion communicates with a smoothly upwardly divergent, circularsectioned trumpet mouthing portion 13 of the fan casing which turnsthrough a full right angle and blends at the crown 14 of the trumpetmouthing portion with a peripheral outwardly extending ange 15. The ange1S has an upwardly directed surface which forms a smooth peripheralextension of the inner wall surface of the trumpet mouthing portion 13and which extends outwardly and downwardly in a gently curved fashionor, at the four corners, radially outwardly and then outwardly anddownwardly in a gently curved fashion to terminate at a peripheral edge15 of generally rectangular contour with rounded corners.

An electric motor 16 is mounted centrally within the base 10 and extendsinto the throat 12 of the fan casing. The motor has a cylindricalhousing which delines with the fan casing at 12 an annular through flowopening 17 which presents the minimum through tiow area of the lairdischarge passage of the ventilator being described. The motor issupported on brackets 20 (see FIG. 3) attached at their inner ends as at21 to the motor and at their outer ends one in each of the four cornersof the base 10 as at 22. Struts 23 are provided, one at each corner,interconnecting the 'base 10 with the underside of the flange 1S tosupport the flange 15 from the base 10. The struts 23 are outside thefan casing.

The motor 16 drives an impeller 24, which is a mixed flow impellerhaving blades 25 of aerofoil cross-section developed along the conicalsurface 26 of a frusto-conical impeller hub 27 carried on the motorspindle 28.

The impeller is mounted and arranged to run with the trailing edges 29of the impeller blades close to the plane of the crown 14 of the trumpetmouthing 13, which plane extends normal to the axis 30 of rotation ofthe impeller. The tips of the blades are shaped to sweep closely afrusto-conical section 31 of the trumpet mouthing portion. The section31 could be contiuously curved instead of being frusto-conieal and theblade tips continuously curved to correspond. However for ease ofmanufacture of the impeller the former arrangement is preferred.

The ventilator is provided with a cowl 32 of inverted shallow-dishedform which is generally rectangular in plan (see FG. 2) but with roundedcorners so as to present a peripheral edge conforming in shape to theperipheral edge of the flange 15. The cowl is supported on four posts 32(see FIG. 3) one carried in each of the four corners of the liange 15immediately over the struts 23. The cowl has a central region which isvery slightly domed for the purpose only of strengthening the structureof the cowl but which may be regarded as substantially ilat, whereby acowl of very low prole is achieved. The cowl has a peripheral edgeportion 33 which is gently curved in a downward direction and thedownwardly directed surface of the cowl portion 33 defines with theupwardly directed surface of th-e ange 1S a confined air dischargepassage 34 extending radially outwardly and downwardly from an annulargenerally cylindrical inlet opening 35 at the crown 14 of the trumpetmouthing portion 13 and surrounding the axis 30 and an annular,downwardly directed outlet opening 36 disposed in a plane normal to theaxis 30 on the side of the crown 14 remote from the central region ofthe cowl.

The ventilator has a damper which operates automatically to close theopening through the fan casing when the ventilator is not operating, toprevent downdraughts and to conserve heat in the building. The damper isgenerally indicated at 41 and comprises a lightweight plate member 42e.g. of thin gauge metal sheet, carrying a peripheral sealing member 43having a sealing lip 44, the sealing member 43 resting on the crown 14of the trumpet mouthing when the damper is closed.

When the fan is operated the plate member 42 floats upwardly on theairstream and is held thereby, pressed against the underside of thecowl. The member 42 is hinged along one edge to link means in the formof two swinging links 45 by hinge arms 46. The arms 46 are pivoted tothe links 45 as at 47 and the links are pivoted to depending brackets 48attached to the underside of the cowl 32 as at 49. Coil springs 5t) biasthe links 45 to the full line position illustrated in FIG. 1 in whichthe damper is closed.

Towards its opposite edge 51, and arranged one towards each side edge 52of the plate member 43, the member 43 is hinged to a pair of struts 55which are in turn hinged one each to a pair of depending brackets 56carried on the underside of the cowl 32. The strut hinge axis extendparallel with the hinge axes of the pivots 47 and 49 and the struts 55themselves extend forwardly, at an angle of about 50 to the plate member43 when the damper is closed.

In the closed position, the plate member 43 is held at a small angle tothe plane containing the hinge axes of the pivots 47 and 49 and must bedisplaced through a position in which it lies in that plane when movingto its open position. When the shutter is closed therefore it is heldclosed by the struts 55 and the shutter member is locked down by anover-center snap, or toggle action since the distance between the linealong which the member 43 is hinged to the struts 55 and the line alongwhich the links 45 are hinged to the brackets 48 is fixed and it isnecessary for the arms 46 t-o fiex in order to allow the member 43 topass through the in-line or over-center position of the toggle.

When the fan is operated the build up of pressure under the plate member43 breaks the toggle along the line of the hinge axis of the pivots 47so freeing the member for upward floating movement to its fully openposition as shown in chain dotted outline in FIG. l.

The springs 50 only lightly bias the member 43 to its closed positionbut are nevertheless sufiiciently strong to break the toggle in theopposite direction and move the damper to its fully closed position whenthe fan is shut down.

The plate member 43 is moved to its open position against the action ofthe springs 50 and the spring force increases slightly with defiection.This is made to compensate for the decreasing angle which the struts 55make with the plate 43 as the member 43 opens a greater component of theair thrust on the front of the plate becoming available to overcome thestronger spring action, whereby the plate member 43 remainssubstantially parallel to its closed position as it moves between itsclosed position and its fully open position. The plate member 43presents towards the impeller a flat or substantially at surface whichis co-extensive with the trumpet mouthing portion 13 of the fan casingso as to confront the whole of the trumpet mouthing portion when in itsopen position, the' member then lying normal to the axis 30.

By placing the impeller in the trumpet mouthing portion 13 of the fancasing an impeller of greater effective diameter may be used comparedwith the case where, as in a conventional powered discharge ventilatoremploying an axial flow fan, the impeller runs in a cylindrical casinghaving a diameter equal to the throat 12. Also the air leaving tips ofthe trailing edges 29 or the impeller blades is free to escapesubstantially radially outwardly over the inner wall surface of thetrumpet mouthing portion towards the crown 14 and a proportion of theair is effectively turned into the discharge passage 34 without strikingagainst the under surface of the non-return shutter member 43 or theunderside of the cowl.

It has been found that by varying the distance of the cowl above thecrown 14 of the trumpet mouthing portion and therefore the gap betweenthe shutter member 43 and the crown when the shutter member is fullyopen the amount of air which can be shifted by the ventilator, all othervariables being equal, can be considerably improved. When the through owarea of the inlet 35 fails to exceed the minimum through flow area at 17by about 33% the volume output of the ventilator is reduced.

Analysis shows that by properly selecting the height of the cowl abovethe trumpet crown a pressure regime may be set up in the space betweenthe trailing edges 29 of the impeller blades and the damper member 43whereby part of the airstream is induced to flow smoothly round thetrumpet mouthing portion to the crown 14 and, after striking against theunder surface of the member 43, the remainder of the airstream can bemade to flow substantially smoothly in a radially outward directiontowards the inlet 35 of the discharge passa-ge 34. In other words byproperly positioning the bafie formed by the damper, in relation to thetrumpet mouthing portion, the airstream may be induced lto change itsdirection from one along the axis of rotation 30 of the impeller intoone radially outwardly of said axis without the use of curved guidingsurfaces to turn the air into the new direction and yet withoutdisturbing the streamline flow of the air yto an extent such as to incursignificant losses in performance. The air is therefore turned eicientlyin a very small height without occupying the space between the crown 14and the cowl 31 with guides and the space may be used to house thedamper means.

The height of the cowl above the trumpet crown 14 also effects thedegree of' expansion of the discharging airstream as it passes throughthe passage 34. By properly selecting the height of the cowl above thecrown 14 together with the shape of the flange extension of the trumpetmouthing and the peripheral edge region of the cowl, it may be ensuredthat streamline flow of air is maintained over the bounding surfaces ofthe passage 34 and accordingly the maximum regain of the kinetic energyof the airstream and the minimum of losses, within the dimensionallimits ofthe passage.

The performance of the ventilator may be increased a little by liftingthe cowl so that the through flow area at the inlet 35 exceeds thethroat area at 17 by an amount somewhat exceeding 33%. In this case therim of the cowl is raised in relation to the crown 14 of the trumpet andthe diameter of the cowl must be increased to maintain theV necessaryprotection against weather entry. Also by lengthening the surfaces ofthe passage 34 the friction losses are increased. There is therefore acompromise to be reached and for aesthetic reasons a small amount ofperformance may be sacrificed by keeping the diameter of the cowl assmall as possible and allowing the discharging airstream to emergethrough the outlet 36 at a slightly higher velocity that might otherwisebe contemplated.

By raising the cowl too far the airstream may break away from thesurface of the ange extension of the trumpet mouthing so creating lossesdue to turbulence in the passage 34.

The ventilator may be designed to operate at a number of fan speeds, theshutter member 43 floating/'freely on the airstream at intermediatespeeds but moving to its fully open position as shown in chain dottedoutline in FIG. l at the maximum fan speed.

The ventilator as described may be used as a wall ventilator and mountedon a vertical wall. In this case the links 45 supporting the non-returnshutter flap would be disposed at the top.

The section 31 of the trumpet mouthing portion may be formed as animpeller shroud and carried round with the impeller, The section 31would in this case be carried at the tips of the impeller blades and runclosely in a gap or recess formed between adjacent stationary sectionsof the fan casing. As a further alternative, the section 31 and thesection of the trumpet mouthing downstream of the section 31 may beformed on and carried round with the impeller as a shroud, a whole -topring of the trumpet mouthing portion or again, the whole of the trumpetmouthing portion rotating with the impeller. The ventilator as describedwith reference to the drawing, using a mixed ow impeller handles moreair for a given size and speed than a centrifugal ventilator and handlesmore air against added resistance upstream of the impeller than aventilator employing a conventional axial flow fan.

It also operates at higher efficiency in the limited confines dictatedby aesthetic considerations and lends itself to more economicalconstruction.

Another advantage is that the shallow, frustoconical impeller operatingwholly within the mouth of the trumpet allows a single plate, dampermember to be used seating on the trumpet flange.

This is an advanta-ge over centrifugal constructions.

Yet another advantage of the construction as described and illustratedis that it facilitates the form of construction in which the impeller ismounted directly on the motor shaft downstream of the motor, sopromoting a very squat and unobtrusive outline for the ventilator abovethe surface of the roof in contrast with the centrifugal design in whichthe motor is usually mounted inside the weather hood downstream of theimpeller.

The cowl 32 in the construction of ventilator described with referenceto the accompanying drawings may alternatively be circular in plan andlikewise, the peripheral edge 15 of the ange 15 may be circular wherebythe cowl and the flange have the cross-sectional shape shown in FIG. 1in all radial planes through the axis 30.

Instead of being carried on the pillars 32', the cowl may be hinged tothe fan casing so that it may be swung back complete with the dampermeans to expose the fan motor unit for servicing.

I claim:

1. In a powered discharge ventilator adapted to be mounted on a buildingstructure, an extraction fan motor (16) having a bladed impeller (25,26, 27) and a fan casing having an air inlet end and an air outlet endsaid fan being mounted in said casing to rotate said impeller therein,and discharge air from the building,

the improvement comprising the combination of said casing including atrumpet mouth portion extending generally axially of said impeller atthe outlet end thereof and daring radially outwardly to extend generallyradially thereof remote from said outlet end to form a surface turninggenerally through a right angle;

with said impeller being mounted within said outwardly flaring trumpetmouth portion and having blades, the tips of which closely match andconform to the shape of said aring trumpet mouth portion to provide foraxial and radial components of air movement over the blade tips, thedownstream edges of said impeller blade presenting the largest radialdistance of said impeller blades, whereby, in operation of theventilator, air will be dispersed radially as well as moved axially, sothat air leaving the tips of the trailing edges of the impeller bladeswill have a large amount lof movement radially outwardly thereofimparted thereto.

2. A ventilator as claimed in claim 4 wherein said confronting surfacesare so shaped and disposed in relation to each other that in operationof the ventilator a sheath of air adjacent the peripheral extension ofthe inner wall surface clings to and flows smoothly along said wallsurface so as to turn through sustantially a full right angle along saidtrumpet mouth without any great loss of energy while that body of airwhich detaches itself from said wall surface impinges on said generallyflat member and flows radially outwardly therealong in all directions,

3. A ventilater as claimed in claim 1 including a generally flat memberspaced from the trumpet mouthing portion of the fan casing andpresenting a substantially fiat co-extensive surface disposed generallynormal to the axis of rotation of the impeller, the trumpet mouthingportion having an inner wall surface which turns through a full rightangle; and said surface of said member being `predetermindly spaced fromthe crown of the trumpet mouthing portion, to constrain air to owsmoothly round the inner wall surface of the trumpet mouthing portion tothe crown thereof without breaking away from the smoothly round surface.

4. A ventilator as yclaimed in claim 3 including means presentingconfronting surfaces, one as a peripheral extension of said surface ofsaid member and the other as a peripheral extension of the inner wallsurface of the trumpet mouthing portion; said confronting surfacestogether defining a confined air discharge passage for streamline flowof the extracted air the through ow area of which progressivelyincreases to an air discharge outlet from said passage to the outsideatmosphere.

5. A ventilator as claimed in claim 4 wherein said air discharge outletis yoffset relative to the inlet to said air discharge passage in thedirection of the axis of rotation of the impeller and said confrontingsurfaces are gently curved between said Ainlet and said outlet -toprevent break away of the discharging airstream therefrom.

6. A ventilator as claimed in claim 4 wherein said generally at memberis a cowl of inverted shallow-dished form, said one of said confrontingsurfaces being formed by the underside of the cowl and said other ofsaid confronting surfaces being formed by an outwardly directed angeportion on the downstream end of the fan casing.

7. A ventilator as claimed in claim 4 wherein said generally flat memberis formed by a damper plate mounted for movement between a closedposition in which it seats on the trumpet mouthing portion of the fancasing and a fully open position in which it lies against the undersideof the cowl.

8. A ventilator as claimed in claim 3 wherein said generally flat memberis spaced from the crown of the trumpet mouthing portion, by an amountsuch that the through flow area of the opening between the crown and theconfronting member exceeds the minimum through flow area for air in thefan casing upstream of the trailing edges of the impeller blades byabout 33%.

9. A ventilator as claimed in claim 1 wherein said trumpet mouthingportion, in the region of said impeller, forms a frusto-conical section,said frusto-conical section blending smoothly with an outwardly vcurvedsection of the trumpet mouthing portion immediately downstream of thetips of the trailing edges of the impeller blades.

10. Ventilator as claimed in claim 1 including a damper plate (41)mounted for movement between a closed position (FIG. 1-full line) inwhich said plate bears against the trumpet mouth of said casing to'close off the air passage therethrough communicating with said fan, andan open position (FIG. l-dash dot line) in which said plate is spacedfrom the axial termination of said casing; means mounting said damperplate for movement between said portions,

the improvement wherein said damper plate mounting means includes astructural member (32) spaced from the axial termination of said trumpetmouth; a first bracket (56) extending from said member toward saidcasing;

a link (55) rotatably mounted in said bracket at one end thereof andsecured to said plate at the other end; said bracket being spaced fromthe axis of said fan and said link being mounted on said plate at adistance different from the axial spacing of said bracket so that saidlink will be included with respect to said axis, as well as with respectto a plane perpendicular thereto; and means moveably securing (45, 46,47, 48, 49) at least one aditional point on said plate to saidstructural member (32) said movably securing means being resilient in aradial direction with respect to said fan.

11. Ventilator as claimed in claim 10 including spring means (50)biasing said plate mounting means to hold the plate in closed (FIG.l-full line) position.

12. Ventilator as claimed in claim 19 wherein said movable securingmeans includes a second bracket (48) depending from said structuralmember (32); a second link (45) connected to said bracket, said secondlink (45) extending in a direction inclined with respect to the axis ofsaid casing as well as with respect to a plane perpendicular thereto;and a flexible interconnection (46) between said plate and rone of saidlinks (45, 55), said exible interconnection permitting resilientdeformation in a radial direction.

13. Ventilator as claimed in claim 12 including spring means biasing atleast one of said links ('45, 55) in a direction towards said casing tothus bias said damper plate towards closed position against said trumpetmouth.

14. Ventilator as claimed in claim 10 wherein said structural member isa cowl (32) covering the trumpet mouth opening of said fan casing, saiddamper plate (41) when in fully open position, bearing against said cowl(32).

15, A discharge ventilator having a casing lirmed with a mouth portionfor discharge of air therefrom; a closure means to close Said mouthportion and prevent backdrafts;

a structural member located in spaced relationship from said mouthportion mounting said closure means for movement between a closedposition, in which said closure means bears against said mouth portion,and an open position in which said closure means is spaced from saidmouth portion, resilient means biasing said closure means in closedposition, and support link means having one end mounted on the closuremeans at spaced locations thereof, said support means having their otherend mounted on said structural member, said support link means bcingmounted on the structural member by a distance which is less than thelength of the links plus the distance between said spaced mountinglocations on said closure means to provide an over-center snap actiontending to maintain said closure means in closed position.

16. Discharge ventilator as claimed in claim 15, wherein said resilientmeans are spring means of suiicient strength to overcome said snapaction in the absence of discharge of air from said mouth portion,whereby said closure means will be closed, and held closed by snapaction, except upon discharge of air outwardly from said mouth portion.

17. Discharge ventilator as claimed in claim 15, at least one of saidlinks yieldingly interconnecting said structural member and said closuremeans to permit said closure means to pass through the in-line positionunder pressure of discharge'of air from said mouth portion.

18. Discharge ventilator as claimed in claim 15, wherein said structuralmember is a bridge member spanning said mouth portion; brackets areprovided secured to said bridge member; and said link means comprisingsupport links having one end of said support links mounted on saidbrackets; said brackets being located diametrically opposite withrespect to said mouth portion, and said links being of equal effectivelength, whereby said closure means, when in open position, will beparallel to said closure means when in closed position.

19. Discharge ventilator as claimed in claim 15, said mouth portionbeing formed as a haring, trumpet-shaped mouth; and fan means disposedWithin said mouth portion to discharge air therefrom in a directiontowards said closure means.

References Cited UNITED STATES PATENTS l1,569,845 l/ 1926 Murphy.2,682,826 7/1954 Wahlberg 98-116 2,875,678 3/ 1959 Shepherd 98432,987,983 6/1961 Solzman 98-43 2,991,927 7/1961 Quick 230-120 X3,012,495 12/1961 Miller 98-85 X 3,324,784 6/1967 Lambie et al 98-43ROBERT A. OLEARY, Primary Examiner.

W. E. WAYNER, Assistant Examiner.

